How often can you apply for the METV in a year and can you stay 9 months in Thailand consecutively?

If immigration thinks you are going to live in Thailand on tourist visas and border bounces you’ll soon be seriously questioned as they will think you are working.

There is nothing written anywhere in Thai law prohibiting this approach. Because there is nothing written in the law about any overall maximum amount of time one can cumulatively stay in Thailand on a tourist visa. Unlike for example what the EU clearly states in law, stating that tourist visa stays can only apply to a maximum of 90 days in every 180 days. Unfortunately, that does not mean that some officer at either the consulate or with immigration does not make up some non-written rule or procedure ... happens all the time with Thai authorities.

What they do however can potentially question is, whether your stay is actually based on tourism. They could assume you are (illegally) working or else questioning where your funds are coming from for such extended stays. In theory, they should have probable cause for that. I real life, they will just deny and it would be up to you to prove otherwise and / or somehow muddle your way through.

Keep in mind: The METV does not grant you 9 month consecutive stay. Used "right" you can squeeze out up to 9 months with border crossings every 60-90 days. So, more so than not being issued a METV every year, your much bigger concern should probably be being denied entry at the border crossing at one point sooner or later - even while holding a valid METV at the time.

Ask the Thai embassy in your home country. You can only get the METV from your home country so the embassy there is who would be able to answer this question.